Semiconductor devices in which the carbon nanotube (hereinafter referred to as the CNT) is used as a channel layer are being researched. The CNT is a carbon material made from a graphite sheet forming a seamless tubular structure. The CNT is about 0.7 nm in thickness at the minimum, and the length thereof ranges normally 1 to several tens μm. Some CNTs are relatively longer, the lengths of which exceed 1 mm. The CNT having the thickness about 5 nm or more exhibits a metallic characteristic, whereas some CNTs having the thickness about 5 nm or less exhibit semiconductor characteristic depending on the way of winding (chirality, helicity) of the graphite sheet. Researches have been proceeding for applications of the CNT exhibiting the semiconductor characteristic to semiconductor devices.
The CNT has electron (positive hole) mobility higher than that of silicon used in semiconductor devices currently in general (for instance, 1,000 to 10,000 cm2/Vs). Such property is highly expected for serving as a switching device such as a field-effect transistor (FET), in particular as switching devices for use in, for example, an image display device to which high response speed is required.
The CNT is also advantageous in view of methods of manufacturing semiconductor devices. A semiconductor device using the CNT can be manufactured by producing the CNT by means of bulk growth methods such as the laser ablation method, the arc plasma method, or the chemical-vapor deposition method, and then depositing the produced CNT on a substrate. Manufacturing the semiconductor device using the CNT does not require a high process temperature, and a transistor with high mobility is thus easily produced.
The advantage that a high process temperature is not required in manufacturing is particularly suitable for satisfying requirements for flat screen image display devices. In requirements for an image display device, the weight reduction of the device itself and the realization of the flexibility which means the ability of being bent or rounded are included. In an active matrix type image display device, polycrystalline silicon is typically implemented as the TFT for driving elements. In a case where polycrystalline silicon is implemented, however, a high temperature (for instance, at or above 300° C.) anneal process is required to increase the electron mobility. Thereby, the process temperature cannot be lowered. In order to use a plastic substrate having excellent flexibility, it is required to lower the process temperature to approximately 200° C. at the highest, and preferably to 120° C. or less. Therefore, for the substrate of an active matrix type display device, not a plastic substrate but a glass substrate, which is heavy and fragile, is currently used. Under such a background, the CNT, which may be almost only one semiconducting material available at a very low process temperature, is expected as a semiconducting material which does not require the high-temperature anneal process.
In addition, active matrix type image display devices using liquid crystal elements or EL elements involve another problem regarding the suppression of the photo leak current. If a light beam enters into a silicon TFT portion of a pixel from a backlight (about 105 lx), the photo leak current increases. In order to suppress this phenomenon, it is necessary to cover a pixel transistor portion with a shielding film made of a metal thin film. However, the aperture ratio is limited by the area occupied by the pixel transistor. This results in reduction of the maximum intensity and the dynamic range of brightness due to the limitation of the pixel aperture ratio, and decrease of the energy efficiency due to the loss of the light in the image display device. Further, a cost-performance ratio is deteriorated by adding the process for producing the metal shielding film.
On the other hand, the semiconductor device using the CNT has a property that the photo leak current is very small (for example, Japanese Journal of Applied Physics, No. 44, Vol. 4A, p. 1592, 2005). Therefore, the CNT is expected as means to solve the problems regarding the cost and the leak current.
In the contexts of those described above, the Japanese Laid-Open Patent Application JP-P2005-93472A discloses a method for manufacturing a field effect semiconductor device using carbon nanotubes as a current path. This method includes the steps of: adjusting the suspension of carbon nanotube; depositing the suspension on a predetermined pattern; and drying the suspension to form a current path including the carbon nanotubes. The Japanese Laid-Open Patent Application JP-P2005-93472A refers to the use of carbon nanotube having the length from 0.1 μm to 10 μm.
When the CNT is applied to a semiconductor device, however, it should be considered that both CNTs having a conductor characteristic and CNTs having a semiconductor characteristic are produced at the same time when manufacturing CNTs. When the produced CNTs are directly used to produce a transistor, the switching characteristics such as the on/off ratio are deteriorated by even only one metallic CNT causing the leak current between the source and the drain (S-D).
Thus, the following attempts have been made: to remove the metallic CNTs from grown CNTs; and to grow only semimetallic CNTs selectively. But, in reality, it is now difficult to remove all metallic CNTs.
For example, when it is attempted to separate metallic CNTs and semimetallic CNTs after their growth, it is difficult to separate them after an aggregation condensation because the CNT is easily condensed by static electricity or physisorption and has very thin and long structure. In addition, it is also difficult to selectively produce the metallic CNT and the semimetallic CNT during their growth. One of the reasons is that the chirality of the CNT, which means the conductiveness or semiconductiveness thereof, is easily changed, even the grain size of the metallic catalyst used for the growth of the CNT is adjusted such that the variation is under 0.1 nm.
Therefore, it is desired to produce a semiconductor device which includes the CNT as a material thereof and having a high on/off ratio even if the CNT contains both of the metallic CNT and the semimetallic CNT.
The selective burning-out technique is known as a method for enhancing a semiconductor device implementing the CNT containing both of the metallic CNT and the semimetallic CNT (Science, No. 292 (2001), p. 706). This technique, however, requires a relatively large current flow for the burning out compared to the operating current of the semiconductor device, which becomes a heavy stress on the whole semiconductor device. Further, it is difficult to process the semiconductor devices individually to achieve stable characteristics. Furthermore, by the selectively burning out, the onstate current of the transistor is unfavorably reduced. Because of these problems, the burning-out technique is not realistic method on industry now.